Don't silence deprecation warning

Deprecation warnings in third-party code are informational. When they arise, do
not silence them (since we treat warnings as errors, these were erroring).

Instead, allow the warnings to go through, as warnings.

BUG=none
R=sdefresne@chromium.org

Review-Url: https://codereview.chromium.org/2341963008
Cr-Commit-Position: refs/heads/master@{#419169}

ios/third_party/gcdwebserver/BUILD.gn

@@ -23,7 +23,7 @@ config("private_config") {
   # TODO(crbug.com/569158): Suppresses warnings that are treated as errors
   # when minimum iOS version support is increased to iOS 9 and up.
   # This should be removed once all deprecation violations have been fixed.
-  cflags = [ "-Wno-deprecated-declarations" ]
+  cflags = [ "-Wno-error=deprecated-declarations" ]
 }
 
 source_set("gcdwebserver") {